The present invention relates to a ceramic-metal junction structure preferably applicable to a ceramic heater or the like.
Metallic layers, containing tungsten (W) as a main component, are conformable with alumina and have excellent heat durability or resistivity. This kind of metallic layers thus can be preferably used for forming signal output terminals or power feeding terminals of electric elements accommodated in a ceramic body. In this case, a conductive member, such as an electric wire, is bonded to the signal output terminal or the power feeding terminal by brazing or the like.
The conductive member may be bonded to the W-containing metallic layer provided on a ceramic body. According to a conventional method, a Ni(nickel)-plating film is formed on the surface of this metallic layer and the conductive member is brazed to the Ni-plating film by using a Cu(copper)-Au(gold) wax material.
According to this conventional method, providing the Ni-plating film on the metallic layer in advance is inevitable in view of the fact that the Cu-containing wax material cannot be directly bonded to the W-containing metallic layer due to poor wettability.
The above-described conventional method is disadvantageous in the high manufacturing cost by the usage of the Cu-Au wax which is expensive.
To reduce the cost, it may be possible to reduce the Au content in the Cu-Au wax. However, reducing the Au content will result in deterioration of the bonding strength. This is believed that the brazing temperature needs to be increased in accordance with the increase of the Cu component. The W--Ni intermetallic compound, produced between the Ni-plating film and the W-containing metallic layer when the brazing temperature is increased, reduces the bonding strength between the Ni-plating film and the conductive member.